Dry investment mold and method



United States Patent Ofiice 3,032,425 Patented May 1, 1962 3,032,425 DRY INVESTMENT MOLD AND METHOD Edward M. Leach, Bunker Hill, Iud., assignor to Union Carbide Corporation, a corporation of New York No Drawing. Filed Feb. 6, 1958, Ser. No. 713,537 7 Claims. (Cl. 106-383) This invention relates to an improved mold mixture and an improved method of precision casting. More particularly, the invention concerns a dry mold mixture for the making of a monolithic mold for the production of products made by the investment molding technique.

By the term investment molding or investment casting, as used hereinafter, is meant a casting process wherein an expendable pattern, normally a wax pattern, is surrounded in the conventional manner with an investment material such as sand, and the entire assemblyis heated to remove the wax, leaving a sand cavity which is the replica of the pattern. This is to be distinguished from the shell molding process wherein a molding material having a thermosetting agent is placed on aheated permanent pattern plate until hardened to form a suitable shell thickness. This shell is mated with a similarly made match shell to form the mold cavity.

The invention is also to be distinguished from the conventional sand casting technique wherein a permanent pattern is used to form the mold cavity.

Several difliculties are encountered in the production of articles made by the presently practiced method of investment casting. Prior to filling the mold flask with the investment molding material the wax or plastic pattern is dip coated'with one or more coatings of a slurry of finely divided sand to provide a smooth surfaced casting. Approximately 4 hours drying time is necessary between each application of coating. This delays the time for producing cast articles and increases the cost of fabrication. In addition, this necessitates control over the environment in order that drying may proceed smoothly and uniformly. A coating which is too wet will wash away when it contacts the wet investment material. On the other hand a coating which is too dry will shrink, become brittle and crack.

Prior to the filling of the mold flask, the final dip coated pattern must dry for approximately 12 hours, further adding to the cost of the finished product.

Another difiiculty involves the use of expensive liquid binder materials for the investment material, which are normally expendable volatiles which can create hazardous working conditions. Moreover, the investment material, because it is in slurry form, tends to create undesirable chemical reactions and requires constant agitation to prevent segregation, entrapment of gas bubbles, and other defects.-

In order that investment material may be utilized with maximum eificiency, it is necessary that these difiiculties be substantially reduced or eliminated.

It is therefore an important object of the invention to provide an improved investment material for making precision casting molds, which is cheaper, requires little or no drying time, and is easily compacted into mold spaces that are not easily accessible, thereby avoiding the entrapment of air or liquids in green molds.

Another object of the present invention is to provide a substantially dry investment material which can be rapidly and conveniently processed into an investment casting mold.

Still another object of the present invention is to provide an investment material suitable for use as a mold in precision metal casting, the mold having one or more suitable bonding agents for imparting strength to the mold during the firing operation.

Other objects, features and advantages of the invention will be apparent from the following detailed description.

According to the present invention, the difiiculties encountered in conventional casting molds are obviated by forming an investment mold from a dry investment material. The term dry material is intended to mean a substantially dry mixture which will not produce undesirable vapors, segregation or gas bubbles. In other words, the mold material of the invention is sutiiciently dry so as not to require the conventional allowance for drying time either after the pattern is dip-coated or during and after the investment procedure.

Investment mixes embodying the principles of the present invention may be made from a dry mix having as a principal component a finely divided refractory or ceramic material, such as sand, fire brick made from fire clay, alumina, or any combination thereof. Included in the mix is a suitable bonding agent. In the preferred practice of the invention, a low temperature bonding agent and one or more high temperature bonding agents are employed, although a greater or smaller number of bonding agents may be used, if so desired.

It is the function of the low temperature bonding agent to improve the strength of the investment mold during the early stages of firing, that is at a temperature between about 300 F. and 1000" F. Low temperature bonding agents which are admirably suited in the practice of the invention include sodium chloride, borax, lead borate glass (glass containing as glass forming oxides PbO, Si0 and B 0 sodium metasilicate and ammonium phosphate, the glass bonding agent being preferred.

At higher temperature levels, above 1000 F., the function of the low temperature bonding agent is supplanted by that of the high temperature bonding material. Examples of effective high temperature bonding agents are feldspar, sodium aluminate, lithium carbonate, and petalite. Feldspar is the preferred high temperature bonding agent.

Preferably, the mixture ingredients for the dry investment mold composition should be within the following composition ranges:

Broad Preferred Ingredient Range, Range,

Percent Percent Refractory -99 -97 Low Temperature bonding agent %8 2-4 High Temperature bonding agent %6 1-2 finely divided ingredients are introduced into a mixing.

vessel in any desired order and thoroughly agitated until an intimate mixture is obtained.

In the practice of this invention an expandable pattern made of wax, plastic or a suitable low-melting metal is prepared in the customary manner. This usually involves assembling several patterns on special wax or plastic runners and gates, dipping the pattern assembly into a ceramic slurry, and stuccoing' (sprinkling) the dipped pattern assembly with a coating of dry sand. The dipping and coating steps may be repeated one or two times if deemed necessary. Thereafter, the dipcoated pattern is suitably mounted in an investment flask and is then ready for the flasking operation. Up to 3 now, a wet investment mix in the form of a slurry has been used to invest the pattern and fill the flask. As far as is known, no attempts have been made to fill the flask with a dry investment mix.

Prior to filling the flask with the dry investment material of the invention, about /2 thickness of a fast setting cement is provided to retain the dry investment material when the base board is removed for the firing cycle. When a ceramic base board is used, no fast setting cement is required because the flask need not be removed from the base board for firing.

In accordance with the present invention, the mold flask containing the prepared pattern is filled with a dry investment material falling within the composition range described above. Filling may be accomplished either by hand or by machine, the latter being preferred because of its rapidity. In this manner, approximately 30 lbs. of dry investment material can be packed in about 5 seconds. A series of sharp impacts applied against the bottom of the mold flask, as by means of a pneumatic hammer, serves to shorten the packing time and decreases the tendency of the differently sized grain particles to migrate. The flask is placed on a vibrating table for about 30 seconds to settle the mix and complete the filling of the flask. The mold is now ready for immediate firing without the necessity of any drying or curing step.

In order that the invention may be more fully appreciated, it may be noted at this point that the usual investment material, in the form of a wet slurry of heavy consistency, requires a long setting time, because of the time it takes to thoroughly dry the mold. For instance, individual tests show that approximately 4 hours of drying time are required after the application of each dip coat to the pattern, and about 12 hours of drying time are required after the final dip coat. After the wet investment has been poured around the prepared pattern, at least 1 hour vibration of the filled flask is necessary to avoid entrapment of air or liquids in the mold. On the other hand, using the dry investment mixture of the invention less than one hour of drying is needed after the first dip coat has been applied to the pattern, no other drying time being necessary. Moreover, this drying time is eliminated if only one dip coat is applied. Approximately 2 /2 minutes are spent in filling the flask with the dry investment material and vibrating the flask. Thus, in the practice of the invention, a saving of approximately 16 to 20 hours in the making of the investment mold may be effected.

Immediately after investing the mold, the wax or plase tic pattern may be removed by heating the mold above the melting point of the pattern, following the method of the lost wax process.

The mold is then gradually heated to the desired casting temperature. At temperatures substantially above the melting point of the pattern, all traces of organic or carbonaceous material from the wax pattern are removed. At approximately 800 F. to 1000 F., the low temperature bonding agents take effect, and above 1000 F. the high temperature bonding agents take efiect to impart strength to the investment casting mold.

In order to indicate still more fully the nature of the present invention, the composition of investment mold mixtures made in accordance with the invention are tabulated below in Table I, it being understood that these compositions are presented as illustrative only, and that they are not intended to limit the scope of the invention. The data reported in Table I are the results of aseries of tests made in accordance with the practice of the invention, in which the mold compositions were successfully fired and sound, precision-made investment casting were produced. Normal rough recovery for these tests were f 95% to 98% (i.e. visual rejects of investment castings constituted between 2% and 5% of the total number cast). In contrast, the rough recovery, using a wet investment mix, usually does not exceed 90%.

4 Table I EXAMPLE I Percent Composition Ingredient Grog (fire brick) 50 mesh.

Grog 20 x 65 mesh.

Bonding sand.

Silica flour mesh.

Silica sand 30 x 40 mesh.

Feldspar.

Ground Glass 200 mesh (lead borate glass).

EXAM P LE II Ingredient Grog (fire brick) 50 mesh. Grog 20 x 65 mesh.

Bonding sand.

Silica flour 100 mesh. Silica sand 30 x 40 mesh. Feldspar.

Sodium chloride.

EXAMPLE III Percent Composition Ingredient Silica sand 30 x 40 mesh. Silica flour 100 mesh. Ground glass.

Feldspar.

EXAMPLE IV Percent Composition Ingredient Grog 50 mesh.

Silica sand 30 x 40 mesh. Bonding sand.

Silica flour 100 mesh.

Sodium Aluminate.

Ground Glass.

From the above description, it will be seen that in vestment casting molds embodying the principles of the invention eliminate most of the problems and difficulties encountered when wet slurries are used in the preparationof investment casting molds. One of the important advantages of the present invention is the unexpectedly short drying period of the dip-coated pattern, which normally requires between about 16 hours and 20 hours drying time when used with a wet slurry. Having no moisture in the starting materials, the mold of the present invention eliminates long mold curing or drying time. This also results in savings in space, scheduling and record keeping. Investment casting molds of the invention can be more economically produced in a continuous production line without the long and expensive drying times so prevalent in the prior art.

Defects such as bubbles, voids or air entrapment, especially in blindspot spaces in the mold which are the customary disadvantages associated with the so-called wet investment mixture of the prior art, are not found in molds made from the present dry investment mixture. Also, since there is no moisture in the investment mixture of the present invention, the usual shrinkage caused by evolution of moisture from the mold is eliminated. In effect, the dryness of the instant investment composition permits the mold to be uniformly and densely packed without segregation of the investment constituents and minimizes the possibility of formation of mold cracks during firing. I

This invention makes possible the reclamation of the investment materials, thereby reducing the cost of materials. In this respect, investment material used in the prior art cannot be reclaimed.

An obvious advantage of the invention is. that it virtually eliminates the cracking of molds from thermal shock during firing. This is because setting of the mold occurs at temperatures well above the critical temperature involving rapid changes in the crystalline form of the mold material.

Other important advantages are the investment material can be prepared well in advance of its prospective use, it has an indefinite shelf life, and possesses no fire hazard. In contrast, wet investment mold material generally cannot be prepared ahead of time and held in storage indefinitely. Also, wet slurries containing alcohol present a fire hazard.

The bonding materials used in the dry investment process are in plentiful supply and of lower costs compared to the limited supply and higher cos-ts of the bonding materials in the presently practiced method of slurry investment casting.

It will be understood that modifications and variations may be effected without departing from the spirit and scope of the invention.

This application is a continuationin-part of my copending application Serial No. 451,287, filed August 20, 1954, now abandoned.

What is claimed is:

1. In the method of manufacturing an investment mold for precision casting wherein a suitably shaped, expendable pattern of low melting point material is covered with a thin shell of refractory material by dip coating the pattern up to two times with a ceramic slurry and stuccoing after each dip coating, and the coated pattern then disposed within a flask and a refractory investment material packed into said flask around said coated pattern to support said coated pattern during and after removal of the expendable pattern material and firing of the mold, the improvement comprising packing into the flask and around the coated pattern, a dry investment composition consisting of from 90 to 99 percent by weight of at least one refractory material selected from the group consisting of sand, fire brick, and alumina, from 0.5 to 6 percent by weight of at least one low temperature bonding agent selected from the group consisting of sodium chloride, borax, lead bor-ate glass, sodium metasilicate, and ammonium phosphate, and from 0.5 to 8 percent by weight of at least one high temperature bonding agent selected from the group consisting of feldspar, sodium aluminate, lithium carbonate, and petalite, and then burning out the expendable pattern and firing the mold to effect bonding of the refractory material.

2. The process of claim 1 wherein the combined amount of high temperature and low temperature bonding agents does not exceed about 10 percent by weight of the dry investment composition.

3. In the method of manufacturing an investment mold for precision casting wherein a suitably shaped, expendable pattern of low melting point material is coated with a thin shell of refractory material by dip coating the pattern up to two times with a ceramic slurry and stuccoing after each dip coating, and the coated pattern then disposed within a flask and a refractory investment mate rial packed into said flask and around the coated pattern to support the coated pattern during and after removal of the expendable pattern material and firing of the mold, the improvement comprising packing into the flaskand around the coated pattern a dry investment composition consisting of about 30 percent by weight crushed fire brick of about 50 mesh particle size, about 5 percent by weight crushed fire brick of about 20 by 65 mesh particle size, about 21 percent bonding sand, about 21 percent by weight silica flour of about 100 mesh particle size, about 19.5 percent by weight silica sand of about 30 by 40 mesh particle size, about 2.75 percent by weight feldspar, and 0.75 percent by weight ground glass, and then burning out the expendable pattern and firing the mold to effect bonding of the refractory material.

4. In the method of manufacturing an investment mold for precision casting wherein a suitably shaped, expendable pattern of low melting material is coated with a thin shell of refractory material by dip coating the pattern up to two times with a ceramic slurry and stuccoing after each dip coat, and the coated pattern then disposed within a flask and a refractory investment material packed into said flask and around said coated pattern to support said coated pattern during and after removal of the expendable pattern material and firing of the mold, the improvement comprising packing into the flask and around the coated pattern a dry investment composition consisting of about 30 percent by weight crushed fire brick of about 50 mesh particle size, about 5 percent by weight crushed fire brick of 20 by 65 mesh particle size, about 21 percent by weight bonding sand, about 21 percent by weight silica flour of about 100 mesh particle size, about 19.5 percent by weight silica sand of about 30 by 40 particle size, about 2.5 percent by weight feldspar, and about 1 percent by weight sodium chloride, and then burning out the expendable pattern and firing the mold to effect bonding of the refractory material.

5. In the method of manufacturing an investment mold for precision casting wherein a suitably shaped, expendable pattern of low melting material is coated with a thin shell of refractory material by dip coating the pattern up to two times with a ceramic slurry and stuccoing after each dip coat, and the coated pattern then disposed within a flask and a refractory investment mate-rial packed into said flask and around said coated pattern to support said coated pattern during and after removal of the expendable pattern material and firing of the mold, the im provement com-prising packing into the flask and around the coated pattern a dry investment composition consisting of about 77 percent by weight of crushed fire brick of about 50 mesh, about 0.5 percent by weight borax, about 20.5 percent by weight silica of 100 mesh particle size, about 0.5 percent by weight ground glass, and about 1.5 percent by weight feldspar, and then burning out the expendable pattern and firing the mold to effect bonding of the refractory material.

6. A refractory mold for investment casting comprising an expendable pattern mold body of low melting material coated with a thin shell of ceramic material by dip coating up to two times with a ceramic slurry and a stuccoing with refractory particles after each dip coating, said coated pattern invested in a flask with a dry investment composition consisting of from to 99 percent by weight of at least one refractory material selected from the group consisting of sand, crushed fire brick and alumina, from 0.5 to 6 percent by weight of at least one low temperature bonding agent selected from the group consisting of sodium chloride, borax, lead borate glass, sodium metasilicate, and ammonium phosphate, and from 0.5 to 8 percent by weight of at least one high temperature bonding agent selected from the group consisting of feldspar, sodium aluminate, lithium carbonate, and petalite.

7. A refractory mold for investment casting comprising an expendable pattern mold body of low melting material coated with a thin shell of ceramic material by dip coating up to two times with a ceramic slurry and a stuccoing with refractory particles after each dip coating, said coated pattern invested in a flask with a dry investment composition consisting of from 90 to 99 percent by weight crushed fire brick, from 0.5 to 6 percent by weight ground lead borate glass, and from 0.5 to 8 percent by weight feldspar.

References Cited in the file of this patent UNITED STATES PATENTS 1,476,001 McIntosh Dec. 4, 1923 2,007,343 Rees July 9, 1935 (Other references on following page) 7 UNITED STATES PATENTS Brenner at al. Mar. 28, 1950 Ossenbruggen et a1. Dec. 7, 1954 Houchins Ian. 10, 1956 Dunlop Sept. 17, 1957 8 Shaul 5 Sept. 17, 1957 Turnbull et a1 Dec. 10, 1957 Moore et a1 Apr. 28, 1959 FOREIGN PATENTS Austria Nov. 10, 1953 

1. IN THE METHOD OF MANUFACTURING AN INVESTMENT MOLD FOR PRECISION CASTING WHEREIN A SUITABLY SHAPED, EXPENDABLE PATTERN OF LOW MELTING POINT MATERIAL IS COVERED WITH A THIN SHELL OF REFRACTORY MATERIAL BY DIP COATING THE PATTERN UP TO TWO TIMES WITH A CERAMIC SLURRY AND STUCCOING AFTER EACH DIP COATING, AND THE COATED PATTERN THEN DISPOSED WITHIN A FLASK AND A REFRACTORY INVESTMENT MATERIAL PACKED INTO SAID FLASK AROUND SAID COATED PATTERN TO SUPPORT SAID COATED PATTERN DURING AND AFTER REMOVAL OF THE EXPENDABLE PATTERN MATERIAL AND FIRING OF THE MOLD, THE IMPROVEMENT COMPRISING PACKING INTO THE FLASK AND AROUND THE COATED PATTERN, A DRY INVESTMENT COMPOSITION CONSISTING OF FROM 90 TO 99 PERCENT BY WEIGHT OF AT LEAST ONE REFRACTORY MATERIAL SELECTED FROM THE GROUP CONSISTING OF SAND, FIRE BRICK, AND ALUMINA, FROM 0.5 TO 6 PERCENT BY WEIGHT OF AT LEAST ONE LOW TEMPERATURE BONDING AGENT SELECTED FROM THE GROUP CONSISTING OF SODIUM CHLORIDE, BORAX, LEAD BORATE GLASS, SODIUM METASILICATE, AND AMMONIUM PHOSPHATE, AND FROM 0.5 TO 8 PERCENT BY WEIGHT OF AT LEAST ONE HIGH TEMPERATURE BONDING AGENT SELECTED FROM THE GROUP CONSISTING OF FELDSPAR, SODIUM ALUMINATE, LITHIUM CARBONATE, AND PETALITE, AND THEN BURNING OUT THE EXPENDABLE PATTERN AND FIRING THE MOLD TO EFFECT BONDING OF THE REFRACTORY MATERIAL. 